I mean, it really isn't hard to write an application, which won't work on Windows or macOS. For example, I have a little utility, which adds a text file into a folder underneath ~/.local/ and opens it in my default text editor via xdg-open, so that I can easily jot something down. Both of things are currently implemented Linux-only.

In this case, I could've pulled in two libraries to do those things with Windows/macOS support. But it's also an incredibly simple application. If you build something more complex, there's a good chance that no library exists and that you still need to make assumptions about the OS.

Of course, a complex applications is likely to be useful enough, that someone wants to use them on Windows/macOS and then contributes support (and pinky-promises to the maintainer to regularly test on those platforms). That's the other vehicle how lots of open-source applications do support a multitude of platforms.

But yeah, it's just not quite as much of a given as your comment makes it sound...